Representatives of Donald Trump Jr. and former Trump campaign chairman Paul Manafort were in talks Friday with Senate Judiciary Committee Chairman Chuck Grassley about their possible testimony next week, a spokesman for the panel told Newsmax.

Grassley also reiterated his threat to subpoena them should they not agree to appear, Judiciary Committee spokesman Taylor Foy said in a statement to Newsmax.

"Chairman Grassley and ranking member [Sen. Dianne] Feinstein have made clear that they intend to hear from all witnesses for next Wednesday's hearing," he said.

Feinstein is a California Democrat.

"Of course, the preference would be voluntary cooperation, however Chairman Grassley has been authorized to compel testimony, if necessary, from the witnesses."

In addition, Grassley had not heard from a third witness sought by the committee, Glenn Simpson, co-founder of Fusion GPS, Foy said.

The Washington opposition research firm produced an unsubstantiated dossier about President Donald Trump's possible ties to Russia.

"Grassley has said he hopes to hear from the witnesses or their attorneys by the end of this week," Foy said.

"Once the witnesses' attorneys have indicated what their clients are willing to do voluntarily, Grassley will consult with ranking member Feinstein on whether to issue subpoenas."

Representatives of Donald Trump Jr. and former Trump campaign chairman Paul Manafort were in talks Friday with Senate Judiciary Committee Chairman Chuck Grassley about their possible testimony next week, a spokesman for the panel told Newsmax.